1. Field of the Invention
The present invention relates generally to a method and a device for modifying an ultrasonic welding horn, and more particularly to a method of modifying the horn to more effectively weld complex thermoplastic parts using ultrasonic welding technology.
Ultrasonic welding of thermoplastic parts is accomplished by creating high frequency vibrations from an ultrasonic welding horn which contacts a surface of the parts being assembled. The vibrations cause surface and intermolecular friction between the parts which produces a sharp rise in temperature at the joints where the parts meet. The rise in temperature becomes great enough to then melt the plastic causing a flow of plastic between the parts. After cessation of the vibration, the material solidifies and a weld results.
In order to make this process more efficient, triangular ridges called energy directors are usually formed on one of the parts are created to contact an adjacent flat area of the other part. Since these energy directors are in direct contact with the other part (leaving a majority of the adjacent surface areas out of contact), they readily become molten when the horn is vibrated causing welds to be localized to the areas of the energy directors.
One problem associated with such ultrasonic welding processes is that when using precision parts having a number of energy directors, certain areas do not receive enough energy from the horn and are thus under-welded. Conversely, other areas receive too much energy resulting in an excess of plastic flow in undesirable areas. In particular, such uneven welding may occur when combining two circular parts with energy directors located at the periphery of the parts. The peripheral location causes a concentration or amplification of the ultrasonic energy which can cause excess plastic flow. If the total energy delivered to the parts is reduced to eliminate the excess plastic flow, there will frequently be insufficient energy delivered to the interior of the parts necessary to effect welding.
Of particular interest to the present invention are circular analytical rotors used for blood analysis having a plurality of cuvettes around their periphery. Often, the energy directors located at the periphery can receive too much energy resulting in plastic flowing into the cuvettes. This is a serious problem since excess and/or uncontrolled amounts of plastic on the interior of the cuvette will impact the effective light path observed in making spectrophotometric analyses with the rotors.
Under-welding of analytical rotors can occur in the interior regions of the rotor having a high density of energy directors. If total energy delivered to the rotor is limited (e.g. to avoid over-welding the periphery), insufficient energy may reach the interior energy directors, causing the interior of the rotor to be under-welded. Consequently, the fluid paths of the resulting molded part may not be sufficiently welded.
For these reasons, it would be desirable to provide improved methods and apparatus for ultrasonic welding of thermoplastic: parts. The method and apparatus should provide for uniform welding of energy directors over the entire surface of a thermoplastic part. In the case of analytical rotors and other circular parts, the method and apparatus should eliminate the problem of over-welding at the periphery, thus insuring a proper light path through the cuvettes in the case of analytical rotors. The method and apparatus should also help insure that areas of the molded parts having a high density of energy directors are sufficiently welded in order to promote proper creation of fluid paths, even when welding at the periphery is not problematic. It would further be desirable if such methods and apparatus were easy to implement and compatible with existing ultrasonic welding techniques and equipment. Particularly, it would be desirable to provide for a simple and inexpensive means to alter the existing ultrasonic horn to obtain the desired uniform welding between the parts.
2. Description of the Background Art
U.S. Pat. No. 4,769,095, describes a method of closing an open ended thermoplastic body with a thermoplastic sheet material through the use of an ultrasonic horn having a cylindrical protrusion extending from its base. This protrusion is used to enter a groove in the thermoplastic body in order to force the sheet material into the groove. After the horn is entered in the groove, the horn is vibrated to achieve a weld within the groove.
U.S. Pat. No. 4,499,845, discloses an ultrasonic horn having a detachable/replaceable end portion which defines the welding surface.
U.S. Pat. No. 4,618,516, describes a method for ultrasonically joining two workpieces having energy directors located on one surface, with an adjacent surface being roughened.
U.S. Pat. No. 4,545,519, teaches the use of a thin metallic "shim" having a hardness greater than that of the workpiece placed over the horn to prevent the horn from sticking to the workpiece.
U.S. Pat. No. 4,430,148, which is typical of several similar patents, discloses the welding of target areas by placing anvils having protuberances behind the materials to be welded.
See also U.S. Pat. Nos. 4,975,133; 4,786,356; 4,757,933; 4,729,778; 4,690,722; 3,993,532; and 3,814,656, which are related to various aspects of ultrasonic welding.